G'day,
the "GP90" range in fact is a bit recent; it was in production from mid 80´s to mid 90´s, using ETA-based movements for a sporty-elegant line.
There was quite a wide variety of models, dials and even functions, including a world timer. But one is a propietory development; the power reserve indication is a development by Girard-Perregaux and basically the same mechanism as in use today.
Personally i like them, even if the size isn´t very "stylish" for a men´s model today and there aren´t many that are as unique as GP´s current models in terms of design or manufacturing challenges. It really was a different era for the brand, therefore i´d personally get one only if particularly attracted by a given design; all are certainly well-made timepieces made to last, just that i find them a little less interesting than what the brand does today.
And if one thing, the "GP90" range pretty much represents the efforts and development of Girard-Perregaux over the past 20 years.
Given almost any of these "GP90" models is at least 10 years old, buying one might be done best with costs of a future service in mind.
Cheers,
Peter